Use empty Stampin' Up! clear mount cases. Stampin' Up! sells them in a package of 4. I cut Designer Series Paper to slip in between the cover and case to give it a decorative look. The DSP was cut at 10 1/2 x 7.
For the inside, I cut strips of self adhesive magnetic sheets and placed them inside the case. Since the dies are metal, they stick to the magnetic sheets perfectly.
Depending on different sized dies, you can place them on the magnetic sheets in any position.
Here is my collection of die stamp cases. Ten cases. I have a lot of dies!